The evolution of modern reforming catalysts and processing techniques, particularly Platforming, from the original concepts embracing noble metals is traced historically by relating the experiences of a number of persons who were involved in advancing the performance of the Platforming process. With the objective of approaching theoretical yields, the formulation of catalysts was continually modified to improve their selectivity and stability at operating conditions favorable for the desired reactions. Experimental data on pure compounds are presented to define reaction mechanisms, and along with data on real naphthas it is demonstrated that the preferred reactions are favored by low operating pressures. With impressive advances in the stability of bimetallic catalysts, sustained operations are permitted at low pressures at which hydrocracking is minimized and dehydrocyclization of paraffins is enhanced, so that near theoretical yields are obtained.
Introduction The Editor of Product RID asked the authors to present an account of signals and their application in a manner which represents the interaction between academia and industry, so the authors have abandoned the usually accepted style of technical chapters. What we hope for here is a tracing of the birth and growth of a process. The chapter could be equally well named “The Pleasures and Frustrations of Industrial Research” or even “The Agony and Ecstacy of Industrial Research” or, in less dramatic terms, “From the Laboratory to the Marketplace”. Under whatever title, this chapter attempts to provide the reader with a factual description of the “raison d’itre” of catalytic reforming and how the Platforming process was accomplished. The authors hope that this chapter will also give the readers from academia a better understanding of in’ dustrial research and will assure the industrial readers that there are many associated common pleasures and frustrations in industrial research. In attempting to bridge the gap between academia and industry, the authors have felt that it is wrong to emphasize “relevance” as a distinguishing feature. Academia finds its work just as relevant as does industry because the reasons for existence and end goals are entirely different. Thus, the “bridging of the gap” really means for both academia and industry to do their jobs well. In academia it is the top notch teachers, and not necessarily the best researchers, who can provide the student with the necessary basic scientific training and arouse their curiosity and intense interest. In industry, it is the group leaders and research directors who can further challenge the curiosity and interest to develop ideas which then lead to new products and new processes. Thus, industrial leaders can be as inspirational as the leaders of the academic community, except that in industry such leaders must develop not only individuals but groups and teams who participate in bringing the new product or new process t o the marketplace. From the scientific and technological viewpoint, this chapter defines understanding of the reactions that take place in catalytic reforming. I t also emphasizes “relevance” in an economic sense, in that a process will succeed only if it realizes a profit for its user. The highest profit in catalytic reforming will be realized when the desired products are made in maximum yield, and in this light the reader will understand the importance of trying to approach the theoretical yields in a practical manner.
Section A. Process Description (M. J. Sterba) In the petroleum industry the word “reforming” is used to designate a process by which the molecular structure of naphthas is changed, or reformed, with the intent of lessening the knocking tendency (or raising the octane number) of those naphthas in internal combustion engines. Also, the reforming process is used to synthesize aromatics-particularly benzene, toluene, and CS aromatics-from selected naphtha fractions.
The antiknock quality of unleaded gasolines is related to the chemical structure of their constituent hydrocarbons. Paraffins, olefins, naphthenes, and aromatics are the four main hydrocarbon types of which gasolines are composed. Normal paraffins have the lowest octane numbers of the family of hydrocarbons, while the isomerized or branched paraffins have much higher octane ratings. I t is well known t h a t the octane number scale has been defined by ascribing a zero rating t o n-heptane which is particularly prone to knocking, and a rating of 100 to isooctane (2,2,4-trimethylpentane), one of the more highly branched octanes. Generally, monoolefins will have higher octane numbers than the corresponding paraffins. Naphthenes or cycloparaffins have octane numbers which are quite high. The aromatics have exceptionally high octane numbers which are in the main over 100. Although the relationship between hydrocarbon structure and knock rating is complicated, these broad generalizations give an indication of the structural changes that reforming processes are intended to accomplish in order to raise the octane number of gasolines. Native hydrocarbon types vary widely in the relative amounts in which they occur in petroleum from different fields; therefore the octane ratings of “straight-run” gasolines will vary. Most straight-run gasolines obtainable by simple distillation from crude oil contain only paraffins, naphthenes, and aromatics and will have octane numbers of 50 or less. As examples, the hydrocarbon type contents and octane numbers of a typical domestic Mid-Continent and of a foreign Kuwait depentanized naphtha are shown in Table I. The octane number of straight-run naphthas, even with the addition of lead alkyls, is too low to permit them to be included in commercial gasolines. Therefore, the chemical composition of these straight-run naphthas needs to be changed, or reformed, so that they can be used in modern internal combustion engines without knocking. Thermal Reforming. The first refining process used t o change the composition of native naphthas to improve their octane rating was thermal reforming, introduced in about 1930. By this process, conducted a t just over 1000 O F and a t pressures in the range of 500 to 1000 psi: (a) olefins were produced from paraffins, (b) high molecular weight paraffins were cracked to produce low molecular weight paraffins having higher octane numbers, and olefins, and (c) native aromatics were concentrated by the cracking of paraffins into much smaller gaseous fragments. Although other reactions were involved, those above resulted in an increase in the octane number up to the vicinity of 85 (Research Method) a t liquid reformate yields which would be uneconomically low a t the present time. T h e creation of aromatics having very high octane numbers was insignificant in thermal reforming; they were concentrated by the destruction of paraffins to gaseous hydrocarbons. I t was the inefficiency resulting from this destruction of paraffins by the thermal process, and the inability to synInd. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
3
Table I. Composition and Octane Number of Straight-Run Naphthas
-
-
~ ____
Hydrocarbon Type
_
Composition, Vol % 48 42 10
Research Octane Number 47 Clear + 3 ml/gal TEL 73
Net HydrOga”
~
Source Mid-Continent Kuwait
~
Paraffins Naphthenes Aromatics
L i g h t Ends
~
_
~
67 22 11 39 67
thesize aromatics, that was the incentive for the development of a catalytic reforming process that could be more selective in promoting the desired reactions and in minimizing the unwanted reactions. The first successful catalytic process that arose from this effort made its commercial appearance in 1949 under the name of the UOP Platforming Process which employed a catalyst containing a noble metal. Process Description. Principal reactions, to be described later, characterizing reforming process employing noble metal containing catalysts are: (a) dehydrogenation of naphthenes to aromatics, to near completion, and with very little ring rupture, (b) isomerization of paraffins to more highly branched forms, (c) dehydrocyclization of paraffins to aromatics, and (d) hydrocracking of paraffins to lower molecular weights, but with a minimum production of light hydrocarbon gases. Because these reactions are conducted in an atmosphere of hydrogen under pressure, no olefins are produced; catalytic reformates are therefore much less susceptible to oxidative reactions and are more stable in storage than thermal reformates. The hydrogen environment in the catalytic reaction zone is created deliberately to minimize the fouling and deactivation of the catalyst by the formation of carbon on its surface. The superiority of catalytic over thermal reforming was demonstrated experimentally in a comparison (Haensel and Sterba, 1951) which showed a reformate yield of 85 vol % for the catalytic process and a yield of only 55 vol % for the thermal process when reforming a Mid-Continent naphtha to a product Research octane number of 85. At this time, various catalytic reforming processes are in use. The simplified flowsheet is described for the Platforming process which was the original of the family employing noble metal catalysts. I t is now the most extensively used process of this group. The flowsheet of Figure 1 shows the reforming unit as it is composed of five principal sections: (1) reactors which contain the particulate catalyst in fixed beds, (2) heaters to bring the hydrotreated naphtha charge and hydrogen recycle gas to reaction temperature and t o supply the heats of reaction, (3) product cooling section and a gashiquid separator, (4) hydrogen gas recycle system, and ( 5 ) fractionation to separate light hydrocarbons dissolved in the separator liquid. The particulate catalyst is contained as a fixed bed in three, and in some cases, a greater number of separate adiabatic reactor vessels with combined feed preheating before the first, and with reheating between subsequent reactions. Because of the rather large endothermic heats of the dehydrogenation reactions, there is a substantial drop in temperature of the flowing reaction stream, particularly in the first reactor in which the rapid naphthene dehydrogenation reaction occurs. Therefore, the effluent from the first and second reactors is reheated t o the inlet temperature required in the subsequent reactor. Usually the charge heater and the inter-heaters are contained in the same furnace housing. 4
TO RECOYery
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1 , 1976
Figure 1. UOP conventional Platforming process.
Effluent from the last reactor in the train is cooled to ambient temperature and led t o a receiver in which the product mixture is separated into a liquid and a gas stream. Most of the separated gas stream, which is largely hydrogen in composition, is compressed and recycled to the reactors to provide the protective hydrogen partial pressure in the reaction environment. A net hydrogen product stream is withdrawn from the system by pressure control on the reaction system. The receiver liquid, containing dissolved light hydrocarbons, is routed to a fractionator to produce a stabilized reformate suitable for blending into finished gasoline pools, and generally free of hydrocarbons lighter than Cg. Most variations in the flow diagram arrangement of the several catalytic reforming processes in commercial use involve the concept of catalyst regeneration frequency. At relatively high pressures and hydrogen recycle gas to naphtha feed ratios, the catalyst is not rapidly fouled and deactivates slowly, so that continuous processing runs varying from a few months to more than a year are attainable. At the end of this uninterrupted processing period the reactor inlet temperature requirements to maintain the desired reformate octane number may reach the limit of heater capabilities, or the catalyst selectivity may diminish to a point where it is economic to terminate the run and restore the activity and selectivity of the catalyst by an in situ regeneration. Regeneration is usually performed in place by burning the carbonaceous deposit accumulated on the catalyst with air diluted with combustion product gases, using the gas recycle compressor to circulate the air/combustion gas mixture through the reactor system a t a controlled burning temperature. This regeneration procedure with proprietary additional steps, can restore the performance of the catalyst to what it was a t the beginning of the preceding processing cycle. An alternative procedure which avoids the down-time necessary t o perform the in situ regeneration is to unload the spent catalyst and reload the reactors with fresh material. The spent catalyst is generally returned to the supplier for recovery of its precious metal content. A t the other extreme is the concept of regeneration frequency in the process designed to operate a t lower pressures and hydrogen recycle gas to naphtha ratios (at which higher reformate yields of a given octane number are obtainable). At these conditions the catalyst fouls and deteriorates rapidly so that frequent catalyst regenerations are necessary. These plants are provided with an additional reactor, so manifolded to the other reactors and appropriate-
ly valved that any one reactor can be taken off-line in turn and regenerated while the others continue to process napht h a feed. A reactor can be isolated from processing service for regeneration as often as once per day. Although higher reformate yields are obtainable by the use of this “swing reactor” design concept, the units tend to be more expensive because of the additional equipment required. Between these two extremes there are available designs and operating techniques that can perform a t any intermediate regeneration frequency, the choice of either extreme or intermediate situation being a matter of that which is economically best suited to particular refining situations. World-Wide Catalytic Reforming. The extent to which catalytic reforming is applied in the petroleum refining industry is shown as daily capacities on Figure 2 for the world, and separately for each of its hemispheres for each year since 1960. Capacities for the Eastern Hemisphere and the world exclude the U.S.S.R. and bloc for which data were not available. During the past 14 years the catalytic reforming capacity of the world has increased by a factor of 2.5, having risen from 2.8 to 7 million barrels per day (B/ D). Most of this increase has occurred in the eastern hemisphere in which the application of catalytic reforming has been catching up, so to speak, with that in the other hemisphere. Along with the general slowdown in overall refinery expansion since 1971, the reforming capacity as seen in Figure 2 has followed this trend in all sectors of the world. Another way of regarding the application of catalytic reforming is by indicating its capacity as a percentage of crude oil processed in a given area. This measure is displayed in Figure 3 for the world, each of the hemispheres, and separately for the U.S.A. About 50 million B/D of crude oil was being processed early in 1973; therefore the 7 million B/D of world-wide catalytic reforming capacity a t this time represents about 14% of the crude. As shown in Figure 3, this percentage has not changed appreciably on a world-wide basis since 1960. The western hemisphere catalytically reforms a greater proportion (about 18%) of its processed crude than the eastern hemisphere (about 12%) in very recent years. As shown in Figure 3, the proportion reformed in the western hemisphere has not changed notably since 1960, but there has been a sharp increase for the eastern hemisphere from 8%to 12%. The plot of Figure 3 shows that the U.S.A. reforms about 23% of its processed crude a t the present time; this proportion represents very nearly the entire native C7 400 OF content of the average crude refined in this area. However, a small amount of U.S.A. reforming capacity is occupied with the processing of thermal and hydrocracked naphthas. While more nearly all of the available naphtha in western hemisphere crudes is catalytically reformed, the average crude processed in the eastern hemisphere is estimated t o contain about 19% of potential reformable naphtha, but only 12% is being reformed. This would suggest that there is a large potential for catalytic reforming capacity expansion in the eastern hemisphere. Contribution of Catalytic Reforming to the U.S. Gasoline Pool. The nation’s gasoline pool is composed of five broad classes of gasolines, defined on the basis of their origin. These classes are: (1) straight run, (2) thermal, (3) catalytically cracked, (4) catalytically reformed, and ( 5 ) alkylate and polymer. Although the national gasoline pool can be resolved into premium and regular grades as actually marketed, and whose proportions have changed with time, it will be considered as a single commodity in this discussion. Only the Research Method octane numbers of the pool and its components will be mentioned, even though the antiknock quality is also reckoned by the refiner, and by the automobile, by Motor Method and Road Ratings.
7r
13-0‘-O
/O
I&
‘‘I&
I962 1963 1964 IdkS 1%6 1%7 I&
P ’
&9
widolr
1970 1971 9l;2
1973
Yeor
Figure 2. Catalytic reforming capacity for the world.
Western rHemisDhere
L
* ~
’
’
~
~
~
1960 1961 1962 1963 1964 I965 1966 1967 1968 I969 1970 1971 1972 1973 Year
’
~
Figure 3. Catalytic reforming capacity as percent of crude capaci-
ty. The changing composition and octane numbers of the nation’s gasoline pool are shown in Table I1 (Sterba, 1971). In Table I1 the unleaded octane number of the pool represents the gasoline quality as produced by the refiner’s processing units, while the leaded octane number represents what is dispensed at the nation’s filling station pumps. Of interest is the dramatic increase of 25 numbers in the unleaded gasoline pool during the past three decades. This has been made possible by the increasing application of catalytic processing during that period. In 1940 the nation’s gasoline pool was essentially a blend of equal ratios of low octane number straight run and thermally cracked gasoline having a moderate octane rating. Catalytic processing was just beginning in 1940. By 1950 the pool gasoline had risen in unleaded octane number from 64 to 75 with the aid of catalytic cracking, alkylation, and polymerization. As Table I1 shows, the unleaded pool octane number rose another 11 units in the next decade from 1950 to 1960, largely by the extensive use of catalytic reforming. During this decade proportion of catalytic reformate in the nation’s gasoline pool rose sharply from a nominal 1%to just over 30%, with much smaller increases in percentages of alkylate and catalytic cracked gasolines. This represents an impressive contribution of catalytic reforming to improving the antiknock quality of the nation’s gasoline pool. Also to be noted from Table I1 is the slight drop in the use of lead alkyls during this decade. This suggests that it was more economical to create octane numbers by catalytic reforming than to use lead alkyls to achieve octane ratings required by the automobile population of the country. During the 1 2 year period from 1960 to 1972, the octane Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
5
~
Table 11. Composition and Quality of U.S.A. Gasoline Pool
1940
Year 1950 1960
Composition, Vol % 50 40 Straight Run 46 32 Thermal Catalytically cracked 2 20 1 Catalytically reformed 2 7 Alkylate & polymer __ 100
100
Research Octane Number Unleaded 64.3 75.3 With lead 74.6 85.1 Lead Content of Pool, g of Pb/U.S. gal. 1.5 2.2
1972
19
12
10
4 38 33 13
31 30 10 100
100
86.2 94.0
89.3 96.8
2.0
2.3
ratings have risen by about 3 units, as shown in Table 11, with all catalytic processes contributing t o this increase. As pointed out in the discussion of Figure 3, nearly all of the available naphtha in the nation’s processed crude oil is being catalytically reformed. However, catalytic reforming can continue to contribute to raising the antiknock rating of the nation’s gasoline pool because by raising the severity of its operation the octane number of the reformate can be increased over a wide range. By contrast, the octane numbers of the gasoline products from other catalytic processes are relatively constant. Thus, the unleaded Research octane number of catalytically cracked gasolines is in the range of 90-93, and that of alkylates and polymer gasoline ranges typically from 92 to 95. On the other hand, the catalytic reforming process is versatile in this respect; in the early 1950’s when the process was being commercialized, the national average unleaded octane number of reformates was just under 85, it is now in the vicinity of 95 as a national average, and there are some sustained commercial operations producing reformates having octane numbers of over 100. Aromatics. In addition to its importance in helping to provide the world with high octane number motor fuels, the catalytic reforming process has been the primary instrument in the synthesis and supply of the basic aromatic building blocks-benzene, toluene, and the Cs aromatics. Of the total benzene consumed about 25 years ago, the portion made by the petroleum industry was of the order of 5%, whereas presently this figure has risen to well over 90%. During this span of years the production growth rate of each of these primary aromatics has been about 10% each year. These aromatics are produced in high purities by the extraction of selected boiling range catalytic reformates largely with sulfolane and polyethylene glycol solvents. Useful By-products. In the manufacture of high quality motor fuel and aromatics, the simultaneous production of hydrogen should be considered a product, rather than a by-product, of catalytic reforming and it is generally profitable to maximize its yield. For the U.S. this production of hydrogen from catalytic reforming amounts to an estimated 2.5 billion cubic feet per day. This hydrogen is useful in the catalytic hydrotreatment of over 4 million barrels per day of a wide variety of stocks. In addition, some of this hydrogen is used for the 3/4 million barrels per day of hydrocracking now in operation. Of the light hydrocarbons made in catalytic reforming, propane finds its way into LPG, isobutane goes to alkylation units, and normal butane to adjust the vapor pressure of finished gasoline. Only small yields of methane and ethane are directed to refinery fuel. 6
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
Section B. The History of the Development of the Platforming Process (Vladimir Haensel) My first introduction to catalytic reforming was in the summer of 1935. I had just graduated from Northwestern University and was to start a chemical engineering course a t Massachusetts Institute of Technology in the fall. Through the efforts of Professor V. N. Ipatieff, who was a great friend of my father’s, I got a summer job a t UOP. I had worked there a t no pay during the previous summers to acquire experience; however, during the summer of 1935 I received $50 a month working as a technician in the catalytic reforming laboratory. In those days our usual catalyst was chromia on alumina which had just been discovered as a dehydrocyclization catalyst. The pressures were nearly atmospheric and no hydrogen was used. When my supervisor left for vacation, Dr. Gustav Egloff came to the laboratory. To those of you who have heard of “Gasoline Gus” but have not met him, I shall give you a short description. A short, wiry, dark-complexioned individual, a champion wrestler in his days a t Columbia, a prolific inventor, with some 300 patents, and the author of a number of books and innumerable articles. He was one of the few individuals who couId explain petroleum refining to a taxi driver and usually did. Egloff used to come to the UOP laboratories in Riverside a t frequent intervals. On hot days, he used to put dry ice in the hollow of his hat apparently to keep a “cool head”. At various technical meetings, he used to appear dressed in all synthetic clothing, and championed the conversion of petroleum to all possible products. On that day in July 1935, “Gasoline Gus” indicated to me that, after all, the main trouble with catalytic reforming as we were doing it then, was the formation of carbon. Young and inexperienced as I was, I realized this also, but then came the bombshell: “You have about three weeks on your own-why don’t you figure out a way of doing the reaction without all this carbon formation?” Frankly, I did not think it was impossible so I worked on it and a t the end of the three weeks there was a singular lack of success. T o this day, I do not know if “Gasoline Gus” was serious about this or not. This was my first brush with catalytic reforming. Then came M.I.T. and in 1937 I was hired by UOP as a chemical engineer, working in the pilot plants. In 1939 came an opportunity to work as an assistant to Ipatieff in the newly established Ipatieff High Pressure Lab a t Northwestern and obtain a Ph.D. degree a t the same time while still working for UOP. In 1941 I returned full time to the UOP Riverside Laboratories to work for Ipatieff. The next five years were most instructive not only from the standpoint of technical growth, but also working for a man who was one of the greatest in the world of catalysis. In retrospect, the greatness was that of a simple man. He knew he was good and he did not have to impress anyone. He was confident because he knew chemistry and was bound and determined to learn more. When Ipatieff first came to the United States he was 62 years old. He knew very little English and the conversations with his associates were most difficult, but chemical progress, nevertheless, was very rapid, despite many comical situations that arose from the language differences. Ipatieff became a U S . citizen in 1935 and the judge asked him if he attended church. Ipatieff said “yes”. The judge persisted: “Which church?” to which Ipatieff replied: “Any church, God is everywhere”. After that reply there were no more questions. Actually, Ipatieff was a very devout person. He was a member of the Russian Orthodox Church. Unfortunately, the Russian church was miles away and the services there lasted two
hours and were not very prompt. The punctual and precise Ipatieff usually went to the Catholic Church a block from the laboratory where the masses were frequent and quite short. In 1950 we flew to The Hague to the Petroleum Congress-this was his first flight, a t age 82. As he said, there was no point doing something small, you might as well fly across the Atlantic. We boarded the plane, sat down, and a t takeoff, the devout Ipatieff crossed himself and left it in God's hands. We did, however, return on the Queen Mary. It has been said that I was Ipatieff's protege and, frankly, knowing the man, I cannot think of a greater compliment. When he decided that I should be on my own, he let go the reins, but was always available for discussion and consultation, and anyone would have been very foolish not to take advantage of that opportunity. The next attempt a t catalytic reforming came a number of years later. We had done some work on demethylation, then drifted into hydrocracking of kerosenes. That required desulfurization and we were delighted a t how neatly kerosenes could be hydrocracked over catalysts like nickelsilica-alumina a t quite low temperatures as long as the stocks were well desulfurized. One of our tests for six-membered ring naphthene content of the gasoline product was the passage over platinum on carbon a t very low space velocities, wherein only these particular naphthenes could be converted into aromatics. Then we removed the aromatics by sulfuric acid treatment and established the five carbon ring naphthene content of specific fractions by a refractive index measurement. Then came the idea to treat an entire gasoline fraction, well desulfurized, over a platinum catalyst. Various supports were made up and, as expected, we did convert a part of the naphthenes into aromatics, but the octane number increase was nothing sensational. When we moved up in temperature, the catalyst lay down and died. So we ran with hydrogen and applied a moderate pressure. The results were not particularly startling but a t least the catalyst survived this ordeal. So we kept moving up, and, sure enough, we did get better conversions. All this time, we were just barely keeping ahead with our desulfurization runs to make enough charge stock for our dehydrogenation runs. One day we ran out of desulfurized stock and used a virgin straight run gasoline as our feed stock. Surprisingly enough, the catalyst did not even bat an eye, but kept right on converting. By this time we were a t about 450 "C and 30 atm pressure and using about 5 moles of hydrogen per mole of feed. This temperature was some 200 "C higher than a t that time recommended for a platinum catalyst. In retrospect, we had wasted a lot of time cleaning u p the feed, but somehow the idea of sulfur poisoning was so ingrained in our minds that the mere idea of using a poison containing stock was unthinkable. A few other things happened; we did notice that now we were converting more naphthenes and that some cracking appeared to take place. The cracking helped to concentrate the aromatics in the product and thus enhanced the octane number. By this time we were doing about as well as had been done previously with a molybdenum oxide alumina catalyst, but with considerably less carbon formation. So now things became more serious, but not serious enough to get people very excited about it. After all, we had been using a 3% platinum on silica catalyst, and even in those days 3% platinum was pretty expensive. Platinum on silica-alumina did much better with respect to octane number but we could not control the hydrocracking very well, SO we switched to alumina which had an intermediate activity. The results looked pretty good, particularly because we could run for days without much loss in activity.
At this point, I was quite fortunate in that Larry Gerhold, who was a t that time manager of the laboratories, took quite an interest in the project and pointed out to me a few facts of life, one of them being that if I were really serious about this I had better start cutting down on the platinum inventory on the catalyst. This we did. I t is surprising what you can do when you have to. We devised all sorts of schemes for incorporating the platinum into the alumina, but one observation was quite critical, and that was that when we made the alumina from aluminum nitrate it was not so good a catalyst as one made from aluminum chloride. This was a real puzzle, until we observed that less washing of the cake made from aluminum chloride made a still better catalyst. Then we found a slight acidity in the exit gas from the unit and the picture began to gel. If chlorine is lost, but is active, then fluorine could be more active and more stable. The first fluorided catalyst gave us one of the highest octane products we had ever obtained. Thus, things really began to look up. You can tell that you have something good and this, a t the time, looked extremely good. This catalyst not only gave a high octane number product, it also had durability, a t least in the relatively short tests that we were able to carry out. Within a year we had about 100 people working on the project-a large pilot plant crew, an.engineering staff, and a substantial research effort, primarily devoted to catalyst manufacture. I t would be only fair to name all the people who contributed to the project, but two persons gave it the greatest push-they were Larry Gerhold and Bob Sutherland. I t has been often said that the most difficult sale is within your own company. Platforming suffered the same fate and it was largely as a result of the efforts of these individuals that the project was not abandoned. One of our troubles was deactivation; in fact, the pilot plant operation did show that we did accumulate carbon on the catalyst. If the initial rate of carbon formation were maintained, the run would not last more than a month or so. This would mean regeneration, and instead of a relatively simple operation, we would have to go to a more complex plant involving cyclic regeneration system and all its difficulties that, a t that time, were most formidable. Fortunately, as we learned, the carbon formation did not continue but leveled out a t an acceptable value. Nevertheless, our troubles mounted. The catalyst requirement was a very pure alumina substrate and that meant that we could not use a commercially supplied material which contained iron and other impurities which seriously impaired the activity. The initial larger scale catalyst manufacture was done by purifying anhydrous aluminum chloride by redistillation and then hydrolyzing it, followed by filtration, washing, drying, pilling, impregnation, etc. Furthermore, misgivings began to arise. We were considering a catalyst which would cost about $10.00 per pound, compared to the usual catalyst cost of about 15-60 cents per pound. The sales department was not too fond of the new process-it was too radical a departure from the processes they were used to selling. The service department, which oversees the commercial operations, was also rather lukewarm about the whole business, and advocated more pilot plant testing and some modifications. A t this darkest hour, the project was rescued by Mr. David Harris, president of UOP. He decided to go ahead. I t is amazing how many people fall in line when the boss makes a decision! I t must have been a difficult decision. Mr. Harris took over the management of UOP in 1945 after it was given to the ACS by the former owners, the major oil companies. Very little cash was available and, a t the time of the gift, the cupboard was pretty bare. Even more disturbing was Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
7
that the basic patents in thermal cracking were expiring, cutting off additional revenue, and, furthermore, the company was facing some 60 million dollars worth of legal suits against it. T o Mr. Harris, a very successful businessman, this was quite a challenge. His number one job was to organize the company operation on a more business-like basis and then settle the legal suits. As he said: “only the lawyers get rich”, and carried this message in his visits to the opposing parties in the suits, and did his very best in staving off possible bankruptcy. When it came to research and development, Mr. Harris knew it was the one substantial asset he must keep intact and make profitable. T o him, Platforming represented a substantial investment but the potential was great and he trusted his advisors. His gamble did pay off and within ten years UOP, already free of legal suits, was sold to the public for about 75 million dollars, thus providing a real bonanza for the Petroleum Research Fund administered by the ACS. Although Platforming was a very substantial contributor to the welfare of the company, the overall efforts in other areas, such as catalytic cracking, alkylation, and other refining processes, also picked up and resulted in considerable income. During the first 10 years after the process was first announced, Platforming units were licensed a t the average rate of one every two weeks, so that by 1960 some 230 units were in operation, representing a capacity of some 1.5 million barrels per day. At the present time, the total licensed Platforming capacity is in excess of 4 million barrels per day with nearly 500 units in operation. We have catalyst manufacturing facilities in the United States, Europe, and Japan. Needless to say, anytime something like this comes along you get a fair number of competitors, so that the total volume of gasoline produced in platinum containing catalytic reforming units is very large. In fact, a t the present time one would be hard pressed to buy gasoline which did not contain a portion processed over a platinum catalyst. You might wonder how the name “Platforming” was coined. In retrospect, platinum reforming shortened to Platforming was a clever condensation and was thought up by Horner Eby, one of the three chemists that comprised our group a t that time. I t was clever because it was descriptive and also implied elevation, and we did mean elevation in octane number and yield. Our public relations department, whose domain was to think up names, took a dim view of the name we used and thought up a number of alternatives, but, like most forced-to-think-up-ideas, they fared poorly and the name had stuck. The name “Platformate” implied the product from the Platforming process and we used it extensively. The Shell Company asked us for the rights to use it in their advertising campaign. The greater mileage due to the use of Platformate was quite real and arose from the fact that when you have more aromatics present the gasoline is more dense and, on a per gallon basis, more total Btu’s are produced on combustion. The first public announcement of the new process in the spring of 1949 was very inauspicious. The “soft sell” was decided upon, probably from necessity, because we could hardly afford an all-out sales campaign. Ed Nelson, a vicepresident of UOP and a great supporter of the process, had been asked to deliver a general paper on recent trends in petroleum technology a t the Western Petroleum Refiners’ Association Meeting in April 1949 in San Antonio. In this paper, as one of five items, he included several paragraphs generally describing Platforming. 8
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
But this proved to be enough. The superintendent of the Old Dutch Refinery a t Muskegon, Mich., Elmer Sondregger, heard the paper and stopped a t UOP’s offices on the way home from the meeting to plead that we install the first unit of the new process a t his refinery. He had a thermal reforming unit which he would make available for revamping to Platforming. The offer was accepted, and the next six months were busy ones around UOP. All records were broken in carrying out the design, purchasing, and construction in this short period. Many laboratory and pilot plant runs had to be made to provide specific answers to problems which arose. Probably the most critical item was the manufacture of commercial quantities of a catalyst which had previously been made only in small laboratory batches. By November, the construction was completed, catalyst supplied, and the unit was ready to start. I was delighted to be asked to attend the start-up that fateful Saturday afternoon. I had been with the company some 1 2 years and during that time, except for a few scientific accomplishments and a number of patents, I had not really contributed to the company’s welfare. So this was the first opportunity to justify some of the investment the company had in me. The unit started up beautifully, but disaster-at least it seemed like a disaster-struck in about three hours on stream, and the unit had to be shut down. What do you do when a disaster, such as this obviously was, really strikes? You retire to a bar. There was only one topic of conversation. In the middle of it, Howard Nebeck, a most capable chemical engineer, left without finishing his drink and retired to his room to do some sketches on the redesign of the reactor. The problem was excessive overheating of the outer shell. I still remember the frequency with which George Thompson, the UOP operator in charge of the unit, was applying the temperature chalks to the outer shell. When the highest temperature chalk promptly melted on the shell, he pulled the switch and the unit went down. I should explain that the catalyst was contained in an inside insulated reactor, and the high temperature insulation was supposed to protect the carbon steel shell from the combined effects of high temperature and high hydrogen partial pressure. Apparently the insulation was not insulating properly. Howard Nebeck’s redesign that night involved a new liner configuration and, seven days later, the unit was modified and brought back on stream, this time to run for about nine months on the same batch of catalyst. The process proved to be exactly the right solution to Old Dutch’s problems. When previously the gasoline from the refinery had been piling up in its tanks with no customers, everyone seemed anxious to try new “platinum treated” gasoline. This market required additional feed stocks and in order to meet this sudden need, the area around Muskegon was scoured for all available naphthas. Among the stocks fed to the unit was a very high sulfur bottoms product from light gasoline sweetening rerunning, and a used chlorinated cleaning solvent. Some of these stocks didn’t run very well, but the unit did manage to survive and some of the limitations of the process were quickly defined. Despite extensive publicity and a good performance, we did not start up a second unit for about two years. In a way this was a good thing. I t gave us a chance to consolidate our position by learning a great deal more about the process and, a t the same time, make some substantial improvements. In addition, the competition was lulled into a false feeling of security and we gained the most important element-time. The chief competitor was fluid hydroforming
and considerable improvements were being made in this process. I remember well a verbal battle a t the World Petroleum Congress a t The Hague between the proponents of fluid hydroforming and ourselves. We had a good thing and we knew it, but they felt the same way and by sheer mass they could and did swing a lot of weight. The next year settled the issue, and sales of Platforming units increased very rapidly. Here again, we were fortunate in getting some of the major refiners interested. I must emphasize that the race continued, primarily due to the pressures exerted from within the company from people like Gerhold, who weren’t satisfied with the status quo. Our catalyst manufacturing method was rather complex and costly. The new method, developed from the very ingenious work by Jim H o e h t r a , made all the difference in the world by providing for a much better catalyst support, made by a continuous process of manufacture. But even this proved to be a difficult thing to put on the marketplace, since refiners who had the old catalyst in their units did not really want to change, so we had to maintain duplicate facilities for a period of time. The performance in the field was good enough to attract imitating competitors. One disturbing occurrence really shook us. A Platforming unit a t Bell Oil and Gas Company was started and ran very well for a few weeks, then reports started coming in t h a t all was not well. The ailment was quite real. The temperature drop across the first reactor declined from the normal 100 O F to about 30 O F and the second reactor took over the reaction. However, this also started to lose activity and the heaters could not keep up with the demand for additional heat to maintain the overall activity. Finally the unit was shut down and the catalyst from the first of the three reactors was returned to us for examination. Here again, we were fortunate in having astute people on our staff, like Jack Murray and E d Bicek of our Physical and Analytical Research. This group developed new analytical procedures which identified arsenic as the poisoning factor, and traced it to the feed stock. The arsenic was present to the extent of only about 30 parts per billion. The platinum in the catalyst just loved the arsenic, even a t these low concentrations, and we were showing good patterns of platinum arsenide. If the unit in the field showed deactivation, we should be able to do so in the laboratory, so we added a few parts per million of an oil soluble arsenic compound to a clean feed and, despite a fairly long run, could not show any poisoning effect. Finally, we realized that in a laboratory unit the surface to volume ratio of the hardware was many orders of magnitude greater than in a commercial unit, and we were undoubtedly picking up the impurity selectively on the walls of the hardware prior to entry into the catalyst bed. As a last resort, we suspended a minute particle of arsenic in the gas phase of the preheater and, sure enough, we deactivated the catalyst within hours. The story of the arsenic poisoning troubles did reach the industry before we had demonstrated a solution to the problem, and some of our newly established competitors were publicizing their platinum catalyst as poison resistant. This created a minor uproar within UOP, and some of the people who got on the Platforming bandwagon by edict were again voicing their misgivings. Before too long the whole thing died down, including the claims of the competitors. I suspect that they had fallen into the same trap t h a t we had by relying upon a laboratory test. Despite all this the poisoning was real-and the only way out was a pretreatment of the feed and this was readily accomplished by a treating step prior to feeding the stock to the Platformer. Thus, we became quite aware of the effect
of possible poisons, and as the octane number requirements were raised we learned to install more sophisticated pretreating facilities. Platinum is a scarce material, and the success of the process depended on development of an economical process for recovering it from spent catalyst. This was very well handled by Herb Appel. On the other hand, catalyst modifications have proven to be a continuing problem. In retrospect, it is amazing how one does not learn from history. When Universal was deeply involved in synthetic cracking catalysts, new catalyst modifications were always tested a t the same conditions that were developed for the initial catalysts, and it is no wonder the new catalysts did not stand much of a chance. We fell in the same trap with the modified catalysts in the Platforming series and a number of good compositions were overlooked for a time because they were tested a t the conditions found best for the reference catalysts. Nevertheless, advances were made and, from the initial R-4 catalyst, as it was called, we have gone through a substantial number of commercial catalysts SO that now we are in the R-30 series with the most sophisticated bimetallic catalysts, the new developments coming largely through the efforts of Ernie Pollitzer and John Hayes and their group. Throughout all this work, one fact has emerged most clearly. You can think up all sorts of catalyst compositions, b u t they are of no avail unless they are properly tested. T h e proper testing implies meticulous attention to the design and details of the test and proper interpretation of the results. I t is people like Rod Donaldson and a number of other most capable chemical engineers who have made this work meaningful and useful for further catalyst development. At the same time, the search continues for the understanding of how these catalysts can be characterized by means other than the catalytic act. We are now finding that such characterization appears to be a reality, in other words, through the work done by the group headed by Hertha Skala, we have developed new insight of the relationship between the metallic components and the substrate.
Section C. The Chemistry of the Process Platforming accomplishes three basic reactions: hydrogen abstraction, hydrogen consumption, and isomerization. The first is exemplified by the conversion of naphthenes and paraffins into aromatics, the second by the hydrocracking of heavier molecules into lighter molecules, while the last is the molecular rearrangement into more compact molecular shapes without change in molecular weight. Even though some reactions are more critical than others, all must be selective since what we are looking for is a maximum yield a t the desired octane level. In any processing scheme it is important to know the theoretical yield. With this in mind we have constructed a model which can tell how closely we approach the theoretical yield. This study revealed some interesting features which influenced considerably the work directed toward the preparation of improved catalysts. In this particular investigation a naphtha composition was assumed as consisting of equal molar amounts of C7, C g , Cg, and C ~ paraffins O and naphthenes. In order to simulate further a straight run stock, a small ratio of aromatics was added to the hypothetical mixture so that the final volumetric composition of the assumed stock is 49% paraffins, 44% naphthenes, and 7% aromatics. The isomeric distribution of the paraffins and naphthenes was assumed on the basis of Rossini’s MidContinent data. Further assumptions include a carbonium ion mechanism of cracking, no cracking of naphthenes, and Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
9
.
1;0_
zDL ,
10
100
,
IO 100 I
,
90 65 80 75 70 C 5 + P l o t f o r m o t e Ystld, L ~ a v i dvolvrne P e r C e n l
95
01 C h o r q l
Figure 4. Theoretical yield/octane study for a Mid-Continent type
stock. a n equal distribution of Cg and Cg ring naphthenes in the charge. It was also assumed that the extent of cyclization of the higher boiling paraffins was greater than that of the lower boiling paraffins, and that the hydrocracking of the higher boiling paraffins was more extensive than the hydrocracking of the lower boiling paraffins. The results of this study are presented graphically in Figure 4. It will be observed that the synthetic stock has a calculated octane number of about 31, which is not too far from what is normally obtained for a 200 to 400 O F MidContinent naphtha. It will be seen that the mere conversion to equilibrium of the naphthenes to aromatics produces a relatively low octane number. The isomerization of paraffins t o equilibrium concentration gives a somewhat higher octane number, while the combination of the two reactions produces a n octane number of around 70 with a Cj+ Platformate yield of 93% by volume. Beyond this point additional octane number increases can be attained by dehydrocyclization and hydrocracking, and the lines shown in Figure 4 give a pattern of the effect of the individual reactions as well as of the combination of the reactions. It will be seen that very high clear octane numbers can be attained by dehydrocyclization, while the hydrocracking reaction is capable of producing t h e higher octane numbersonly a t greatly reduced yields. In actual practice, the yield pattern is fixed somewhere between the dehydrocyclization and hydrocracking lines. In view of the fact that the reactions are not so selective as calculated, and that side reactions such as the formation of methane and the cracking of naphthenes d o occur, there is a further shift in the direction of a poorer yield/octane relationship. However, the information presented above does give a semiquantitative picture of what happens in catalytic reforming. Another interesting feature brought out by the above study is that a substantial portion of the higher boiling paraffins is removed from the stock by virtue of greater susceptibility t o both dehydrocyclization and hydrocracking with the result t h a t additional conversion has to be performed on the lower boiling paraffins produced by hydrocracking where the dehydrocyclization becomes more difficult. Furthermore, the hydrocracking of the lower boiling paraffins can easily result in the formation of two fragments, both of which are butane or lighter and, therefore, cannot be included in the Cg+ gasoline yield. T h e problem 10
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
95
C,+
90
. _ ,
05 I
.
75
BO
roI
P l o l l o m o t e Yield, Liquid Volume Per Cenl 01 C h l r g e
Figure 5 . Theoretical yield/octane study for a Middle East type stock.
thus becomes one of attaining maximum octane numbers without too great a sacrifice in yield due t o excessive hydrocracking. A similar yield octane study has been made on a lower naphthene content synthetic stock, in this case simulating the Middle East source. T h e results are shown in Figure 5. It will be observed that in order to attain high octane numbers with this stock, even more selective paraffin conversion has to be achieved. T h e theoretical yield/octane study assumes that we obtain equilibrium concentrations of the products when we dehydrogenate naphthenes and isomerize paraffins. From a kinetic point of view, equilibrium concentrations are approached hut not fully attained unless very low space velocities are utilized. Thus, we are faced with a problem of establishing some idea of the relative rates of the major reactions which take place and adjusting our catalyst compositions and operating conditions accordingly. For example, if hydrocracking were very rapid and dehydrocyclization were very slow, we could not expect to obtain a respectable yield/octane relationship. On the other hand, we d o know that dehydrocyclization is a hydrogen producing reaction:
While hydrocracking is a hydrogen consuming reaction:
C-C--C-C---C--C--C
+
H-
+
C
I
c4-c
+ c-c--c
Thus, it is apparent t h a t for the same catalyst composition lower operating pressures will accentuate dehydrocyclization and deemphasize hydrocracking. Indeed, the modern trend is in the direction of lower pressures permitted by the development of more active and more stable catalysts. But what about catalyst composition? From the beginning of the work on dual-function catalysts, that is, those that contain a n acid function in addition t o the dehydrogenation-hydrogenation function, we have been concerned with the proper balance of the two functions. A definition of the two terms is now in order. We believe that hydrogen
abstraction or hydrogen addition is catalyzed primarily by the platinum component of the catalyst. On the other hand, any structural rearrangement involving the making or breaking of carbon-carbon bonds is catalyzed primarily by the acid component of the catalyst. When the cyclohexane is converted into benzene:
no carbon bonds are rearranged or broken and hydrogen abstraction is the only reaction. Thus, the platinum component is the only one required for this reaction. However, when methylcyclopentane is converted into the same end product, benzene, it is obvious that carbon-carbon bonds are broken and new ones are made, and along with that hydrogen abstraction occurs:
CH? ,CH, H , C n C H H,C
CH
CH?
CH thus requiring the presence of an acid function in addition to the platinum. We mentioned earlier that chlorine and fluorine act as suitably acidic components of the catalyst. Let us examine in more detail how this activation can occur. T h e catalyst surface is largely that of a high surface y-alumina, a polymeric structure with an occasional hydroxyl group. I t was originally assumed that the acid sites on alumina were protonic (Bronsted) sites localized on the hydroxyl groups t h a t were known to remain on an alumina surface even after calcination:
?H I
II
-iUIo/*q
-
I
I
The effect of halogen on acidity was believed t o be an inductive effect: 741,
,AI\
.o v
-Al,
A , l, 0
f
A succession of studies threw some doubt on the validity of this picture. Measurements by Hall, Hirschler, and others appeared to indicate t h a t most of the acid sites on alumina were Lewis acids. The evidence by no means rules out the presence of some protonic sites. The presently accepted view was first proposed by Weller and later elaborated by Peri on the basis of infrared evidence. In this picture the acid sites are oxygen bridges formed by dehydration of neighboring OH groups during calcination. These bridges can open heterolytically to provide paired acid base sites:
The presence of a neighboring halogen would enhance this polarization by an inductive effect: F
G
The exposed Al+ ions will normally react as Lewis acids although in the presence of HC1 or HzO it is conceivable t h a t protonic sites can be formed. There is some evidence that the oxygen bridges remain in a closed (neutral) form until a reactive molecule actually approaches the site. Let us now examine how this change in acidity affects the conversion of methylcyclopentane which must undergo a carbon-carbon break on the way to benzene. In the first instance, we have converted methylcyclopentane with catalysts containing 0.3% platinum on alumina, but variable amounts of fluorine. The conditions were 260 psig, 4 liquid hourly space velocity and a hydrogen:hydrocarbon ratio of 6 and a temperature of 500 "C:
% Fluorine 0.05 0.15 0.30 0.50 LO 1.25 31.5 41 59 71 71.5 %Benzene 25 I t will be observed that a t a very low acidity, which could be considered as the intrinsic acidity of the alumina, a small amount of benzene is formed. Furthermore, as the fluorine content is increased from 0.05 to 0.50% the conversion increases linearly with fluorine content. At fluorine contents in excess of 1%no further increase occurs since an equilibrium value is reached with respect to benzene formation. Now let us examine the effect of platinum concentration a t a fixed fluorine content. The catalysts in this instance contained 0.77% fluorine and platinum was varied from 0.012% to 0.30%. The conditions were the same as for the study of the fluorine effect. The following results were obtained: %Platinum 0.012 0.03 0.05 0.075 0.10 0.15 0.20 0.30 %Benzene 14.5 45 56 63 63.5 63 63.5 63 I t will be noted that beyond 0.075% Pt, no further conversion to benzene is attained, and the conversion a t that point is limited by the fluorine content. I t is truly amazing t h a t such a low level of platinum can induce the transformation. The data on the fluorine effect and platinum effect clearly indicate the remarkable cooperative action of a dual function catalyst, and begins to give us an idea of the course of the reaction when methylcyclopentane is converted into benzene. How do we establish the mechanism of the reaction? One time-honored scheme is to examine the concentration of reaction products as we change the contact time. If a t a shorter time product B increases in concentration relative t o product C, we can conclude that product B is formed as a precursor to product C. In the conversion of methylcyclopentane the following reaction is proposed:
Charge
B
C
as can be observed from the experiments in Table 111. The ratio of methylcyclopentene to benzene increases as the space velocity is increased, indicating that methylcyclopentene is a precursor of benzene. However, cyclohexene, the expected intermediate between methylcyclopentene and benzene, has not been found in the product. The other point is that even a t high space velocities some paraffinic material is formed through the hydrocracking reaction, leading to the formation of n-hexane and isohexanes. We believe t h a t this reaction of ring opening is catalyzed t o a larger extent by the platinum component and to a lesser extent by the acid component. Thus, in this inInd. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
11
Table 111. Conversion of MCP over Pt-Al,03-Halogen
LHSV
MCP reacted, (mol % of MCP charge)
Catalyst 0 Conversion of MCP t o products (mol % of MCP reacted) _~~_~______. __-_ __
--___-__
MCP=
Bz
CH
Paraffin
Cyclopentane
Mol ratio [ MCP'lABz] x 10'
10 62 1.5 56 1.3 40 1.6 15 52 2.8 55 1.2 39 2.5 30 33 7.1 53 3.1 36 1.4 60 19 12 50 3.6 30 3.3 120 14 16 50 3.5 30 (blank) 0.8 a General conditions: 300 psig, 520"C, 6 : l mol ratio H,/charge, charge stock, 25:75 mol % MCP:Bz.
2.7 5.1 13 24
32
-
Table IV. Conversion of CH over Pt-Al,O, Catalysts" ~~
LHSV
CH reacted, (Mol % of CH charge)
Conversion of CH to products (mol % of CH reacted) CH=
Bz
____-
MCP
Paraffins
0.1 99.4 0.5 0.0 0.7 98.7 0.6 0.0 1.7 97.3 1.0 0.0 2.3 96.8 0.9 0.0 2.7 96.2 1.1 0.0 2.8 95.8 1.4 0.0 00 (blank) 0.0 0.0 0.0 0.0 0.0 UGeneral conditions: 300 psig, 520°C, 6 : l mol ratio H,/charge, charge stock, 50:50 mol % CH:Bz.
1000 2 000 4 000 8 000 1 6 000 32 000
32.0 27.5 18.0 9.1 5.5 2.9
stance, it is largely a side reaction and not a part of the sequential reaction involved in the conversion of methylcyclopentane to benzene. Now that methylcyclopentene has been established as a n intermediate, we should consider its conversion t o benzene. T h e most likely route is:
You have noted that both cyclohexane and benzene are found in the product, but no cyclohexene. T h e thought now occurs that if Kz and K3 are both very high in comparison t o K1, there is virtually no chance of observing cyclohexene since it is used u p as quickly as it is formed. In fact, it was shown by Ipatieff in the 1930's that cyclohexene disproportionates into cyclohexane and benzene a t 80 "C over a nickel-on-kieselguhr catalyst. At that point, one begins to wonder if cyclohexene is a n intermediate in the conversion of cyclohexane to benzene. If this is the case then we can again apply the method of reducing the contact time in our search for the intermediate:
O=O==O
It is apparent t h a t cyclohexene did not appear as a n intermediate a t space velocities of the order of 120 when methylcyclopentane was the reactant; thus we must think in terms of space velocities that are orders of magnitude greater. Also, we have t o do a little catalytic adjusting, which implies that we must not use a catalyst at this stage of investigation which could induce the isomerization of the intermediate cyclohexene to methylcyclopentene. In other words, we want a dehydrogenation catalyst. T h e results are given in Table IV. You will observe that as we increase the space velocity from 1000 to 32 000 there is a definite trend in the direction of increasing cyclohexene in the product, particularly if one looks at the ratio of cyclohexene t o incremental ben12
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
Ratio [CH=/A Bz] x 103 1
7 17 24
28 29
-
zene. We should point out that experiments at these very high liquid hourly space velocities are not easily carried out, and a great deal of chemical engineering ingenuity and effort have gone into devising the equipment and method of operation. The same thing applies t o the analytical techniques for the detection of the components in the product. Note that some methylcyclopentane is also formed. This would be expected, since we still have intrinsic acidity of the alumina and when cyclohexene is present it can isomerize into methylcyclopentene and then be hydrogenated into methylcyclopentane. Now the whole mechanism picture is beginning to gel and cyclohexene thus becomes the focal point from which different products may be formed:
o==fJ==o it
6 4 We realize that the catalytic dehydrogenation of cyclohexane to benzene is a n extremely rapid reaction, since at space velocities which are some 10 000 times greater than those used in commercial practice we are still obtaining discernible conversions. But even under these conditions, the formation of benzene from the intermediate cyclohexene is still rapid; otherwise we would produce more cyclohexene. Is it possible t o slow down this second reaction? We d o know that sulfur is a potent poison for platinum in hydrogenation-dehydrogenation catalysis. If we now assign some rate constants to the overall reaction, can we change some of these by poisoning? This means that in the reaction:
Table V . Conversion of CH over Pt-Al,O, Catalystsa ____-
LHSV
S
N
200
50 50 50 50 50
0 0 0 0 0 0
800 1600 4 000 8 000 8 000
(mol % of CH charge) 72 47 19
4.4 1.0
CH=
Bz
MCP
0.7 0.9 1.5 4.2
65
32
57
48 27
18 (--) 2.3 97 a General conditions: 300 psig, 520 “C, 6 : l mol ratio H,/charge, 0
9.1
MCP’
Paraffins
1.5 5.0 Trace 42 8.7 0 45 24 0 44 38 0 0.9 0 0 charge stock, 50:50 mol % CH:Bz. 1.0
37
[CH=/A Bz] x 103 11
16 31
155 m
24
Table VI. Conversion of Methylcyclopentanea Sulfur, ppm Products, mol % Benzene Cyclohexane a 4 8 5 ‘(2, 300 psig, H,:HC
6 24.5 0.6 =
30
24.6 0.6
4, Pt-Al,O,-halogen
100 50 22.8 23.9 0.6 0.8 catalyst, 20 LHSV.
300 24.0 0.9
1000 24.5 2.1
2000 18.4 2.4
3000 13.1
4.2
The unpoisoned relative rates are:
Kz >> K1>> K3
< K4
However, the poisoned relative rates may be quite different. Table V shows the results obtained when we add 50 parts per million of sulfur in the form of thiophene to the previous charging stock. The results are most striking. The net conversion of cyclohexane has been appreciably slowed down, by probably as much as one order of magnitude. Second, a t still moderately high conversions, methylcyclopentane (MCP) and methylcyclopentene (MCP’) constitute a greater part of the product than benzene. Next, cyclohexene is present in substantial concentrations, and the ratio of cyclohexene t o incremental benzene reaches very high values and finally infinity as no net benzene is made a t the highest space velocity. One should also note the fact that at the lowest space velocity where a large amount of MCP is the product, only a small amount of paraffin is produced, indicating an essentially complete suppression of the ring opening reaction. Finally, the concentration of MCP= is in excess of equilibrium relative to MCP, indicating that the route is from CH= to MCP= to MCP and the reaction of MCP= to MCP is kinetically hindered. Now we can express the effect of sulfur on the rate constants in the reaction:
Unpoisoned: K2
> K4, K3 > Kz.
>> K1 >> K3 < Kq; poisoned: K Z < K1, K3
We see an almost complete reversal of the original set of rate constants. Thus, the presence of only 50 ppm of a poison has changed the course of the reaction, and one can imagine the conclusions we would have drawn regarding the overall reaction had our original charging stock been contaminated with what appears to be a small amount of sulfur. This reversal of rate constants due to the presence of sulfur has an interesting sidelight. By now we are well familiar with the conversion of methylcyclopentane and cyclohexane, but let us look a t it in a little different light:
“‘1 t
n v where K3 > K4 = K 1 > K z . Now, if we just consider the significance of these rate constants, we can conclude that the conversion of methylcyclopentane should not be affected by sulfur until K3 becomes smaller than K P .The results shown in Table VI confirm this. You will note that the yield of benzene remains virtually unchanged until in excess of 1000 ppm of sulfur is added t o the methylcyclopentane. A t this point the rate of cyclohexene conversion drops below the rate a t which methylcyclopentene is isomerized to cyclohexene and a net reduction in benzene occurs. Also, one should note the rise in cyclohexane concentration, since the cyclohexene to benzene conversion is poisoned to a greater extent than the conversion of cyclohexene to cyclohexane. There was, however, one set of experiments, the results of which we did hypothesize in advance. The reasoning was as follows: the sulfur poisoning of the reactions involving cyclohexane does produce a substantial reduction in the rate of conversion of cyclohexane and, in particular, slows down the conversion of cyclohexene to benzene. The slowing down of this last reaction accentuates the participation of an otherwise slower reaction, that is the conversion of CH= to MCP’ and then to MCP. This reaction, as pointed out before, is acid catalyzed. The acid function is most likely a combination of the intrinsic activity of the alumina and the presence of sulfur. Thus, if we were to poison the acid function we should no longer observe the substantial formation of MCP= and MCP despite the presence of sulfur. The experiments listed in Table VI1 were carried out in the presence of 50 ppm of S and 10 ppm of N, added as pyridine. Before we comment on the effect of the additional poison, let us make a comparison a t approximately the same extent of conversion of cyclohexane (see Table VIII). The effect of the 10 ppm of N to the already sulfur poisoned system is quite startling. We have suppressed completely the acid-catalyzed reaction of cyclohexene, so that the “normal” reaction to benzene is once again the predominant reaction. However, what we have done now is to allow Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
13
Table VII. Conversion of CH over Pt-Al,O, Catalysts Wt, ppm in charge LHSV
S
N
CH reacted (mol % of CHcharge)
800 1600 2 400
50 50 50
10 10 10
21 10 6
Conversion of CH to products (mol % of CH reacted) CH=
Bz
6 12 17
92 84 82
MCP
MCP'
Ratio [CH=/ABz]
x
Paraffins
2 0.3 3 0.8 1 0.2 UGeneral conditions: 300 psig, 520 "C, 6 : l mol ratio H,/ charge, charge stock, 50:50 mol % CH:Bz.
0 0 0
lo3
65 143 207
Table VI11 W t ppm in charge
LHSV
S
N
CH reacted (mol % of CH charge)
1 6 000 4 000 2 400
0 50 50
0 0 10
5.5 4.4 6
Conv. of CH to products (mol % of CH reacted) CH-
Bz
MCP
MCP'
Paraf.
2.7 4.2 17
96.2 27 82
1.1 45 1
0 24 0.2
0 0 0
cyclohexene concentration to build up by blocking the acid catalyzed reaction by the nitrogen poison, and blocking of the cyclohexene conversion to benzene by the sulfur poison:
The price we had to pay for blocking the two reactions is t o slow down the primary reaction of cyclohexane to cyclohexene by a substantial factor. This can be observed by comparing the space velocities required to give approximately the same absolute conversion of cyclohexane. Sulfur poisoning reduces the absolute rate by a factor of 4, while the combined effect of sulfur and nitrogen reduces the absolute rate by a factor of about 7. What does all this mean? First, we now have a much better understanding of dual function catalysis. Second, we have mastered a rather difficult chemical reaction and have been able to change its course a t will. But most important, this information helps us to develop new catalysts and new reactions and it is difficult to ask for more. Nevertheless, in a way, we have just scratched the surface. We do need to know a great deal more about dehydrocyclization of paraffins and hydrocracking of paraffins. The information on conversion of naphthenes has helped in establishing the course of reaction once the paraffin has undergone a ring closure, but the initial mechanism, presumably involving a conversion to an olefin and then a self-alkylation reaction, is not clearly established. And yet, you will recall that in our theoretical yieldloctane study the dehydrocyclization reaction is the most efficient reaction for octane number improvement. How can we improve the efficiency of the dehydrocyclization reaction? We can think of many possible ways, but one factor is particularly important. Dehydrocyclization has to compete with hydrocracking and, as pointed out before, the first is a hydrogen producer, while the second is a hydrogen consumer. Thus, a reduction in operating pressure will favor dehydrocyclization and disfavor hydrocracking. This point is well illustrated in Figure 6. I t is quite apparent that a t a constant octane number we can reduce the amount of hydrocracking substantially by reducing operating pressures. If we don't hydrocrack, and the remaining paraffins are already isomerized to an equilibrium value, we make up the difference by increasing the 14
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
Ratio [CH=/A Bz J
1301
x
lo3
28 155 207
Hydrogen
--_------o
-
-----0--
-
-
-
__
-
u 100 200 300
700
~--
-
400
500
600
Pressure, psig
Figure 6. Platforming 178-399
O F
naphtha to 98 Clear RON.
extent of dehydrocyclization, since we are maintaining a constant octane number. All we really need to look a t is how little weight percent yield is lost by making hydrogen and how much is lost by making C1, Cz, CB, and Cq. We should mention that although the difference is very great, it is slightly offset by the fact that since there are weight yields, we do obtain a somewhat smaller yield by volume because the aromatics are more dense than paraffins. The motorist gains, however, since he obtains more Btu's on combustion of the more dense fuel. In the example just given the original feed stock was a mixed Middle East naphtha with a paraffin content of 64% by volume. When we process a 181-295 O F light Kuwait naphtha containing 71% by volume of Cs, C7, and Cs paraffins, an even more pronounced effect of pressure is obtained (see Figure 7). You will note that for the same 98 clear research octane number product, the lighter charge stock has to be processed a t 100 psig pressure t o obtain the same yield as the heavier charge stock a t 200 psig. This is to be expected, since with smaller molecules it is not only more difficult to cyclize, but, in addition, the hydrocracking reaction of the smaller molecules produces a larger proportion of fragments in the C I - C ~range. A further consideration of the effect of pressure on dehydrocyclization is the finding that pressure has an adverse effect on the yield of aromatics having the same number of carbon atoms as the parent paraffin. This is quite apparent from an inspection of the results obtained upon processing normal nonane a t two different pressures (Table IX).
Table X % of Total C,-C,
Product, wt % of Charge CH, C,H, CJ-4 i-C,H, n-C4
0
Total
1.2 2.9 4.0 2.5 2.6 13.2
"l>
31.1%
2)
68.9%
22.0
19.7 100.0
Table XI. Hydrocracking of n-Heptane
!
k,
n-Heptane decomposed Product distribution, mol %
17.8
%
"
70 I
100
200
0.2 0.2 40.5 45.2 9.6 4.3
300
Pressure, p r i g
Figure 7. Platforming light Kuwait naphtha to 98 Clear RON. Table IX. Reforming of n-Nonane, 950 O F , 1.5 LHSV
100.0
Product distribution, wt % Nonaromatics Pressure C,-C, 100 10.9 300 21.5
Aromatics
C,+
C,
19
1.6 2.0
20
c,
C, 3.1 5.8
6.2 10.1
c 9
54.5 36.4
In both instances, essentially all of the normal nonane is converted, but the aromatic distribution is quite different a t the two pressures, the higher pressure producing substantially less Cg aromatics and more c 6 - C ~aromatics. In addition to the hydrocracking of the paraffin to produce lower paraffins, we observe demethylation and dealkylation of the Cg aromatic. This latter side reaction further reduces the overall yield. Despite the problems due to hydrocracking and dealkylation of aromatics, the fact remains that a t 100 psig the yield of aromatics from n-nonane a t 950 O F and 1.5 LHSV is about 70% by weight. Just consider how complex the reaction must be which in a relatively short time proceeds to give:
C
